The mucopolysaccharidosis (MPS) diseases are caused by hereditary enzyme deficiencies which result in multi-organ pathology. Onset is usually in childhood; mental retardation is a common feature. One of these diseases, MPS VII, is caused by a lack of the lysosomal enzyme beta- glucuronidase (GUSB). The avail-ability of a mouse homologue of MPS VII and a variety of methods to detect GUSB activity have been valuable tools for gene transfer experiments because both the degree of gene transfer and the therapeutic efficacy can be assessed in the same mice. Based on our new preliminary data, we propose experiments to rigorously test one of the most promising tools for gene transfer into the CNS, the lentiviral vector, using four carefully designed vectors for in vivo and in vitro experiments. The Specific Aims of this revised research proposal are to: (1a) Quantitate GUSB enzyme activity in cultured neurons transduced by lentiviral vectors; (1b) Quantitate biochemical changes in GAG turnover in cultured MPS VII neurons before and after vector correction; (2a) Quantitate expression from four lentiviral vectors in vivo and identify transduced cell types; (2b) Evaluate whether long-term CNS transgene expression can be achieved with lentiviral vectors; and (2c) Determine the extent of correction of pathology by GUSB-encoding lentiviral vectors in the MPS VII mouse brain.